Endochitinases are widely distributed among higher plants, including a number of important crop species. They are generally considered to be involved in plant defence against potential pathogens. We have cloned a class IV chitinase gene (AtchitIV) from Arabidopsis thaliana. Southern blot analysis allowed the detection of two cross-hybridising genes in the A. thaliana genome. AtchitIV transcripts are detected in seedpods, but not in roots, inflorescence stems, leaves and flowers of healthy plants. The transcripts accumulated very rapidly in leaves after inoculation with Xanthomonas campestris. Maximum mRNA accumulation was reached one hour after infection and decreased to very low levels 72 hours after induction. This result suggests an involvement of AtchitIV in the initial events of the hypersensitive reaction. Nevertheless, A. thaliana plants transformed with the gus gene under the control of a class IV chitinase bean promoter, showed GUS activity in seed embryos. These data, together with the constitutive expression of the endogenous gene in the seedpods, points to additional physiological roles for this protein.
The heavy metal lead was administered to Arabidopsis thaliana plants by foliar spray. At a concentration of 14mol m(-3), the lead nitrate suspension induced densely distributed necrotic lesions on A. thaliana leaves. A number of Arabidopsis ecotypes were tested and a differential response to heavy-metal toxicity was noted, The necrosis provoked as a result of the phytotoxic effect of lead had a similar appearance to the necrotic lesions observed in a hypersensitive response of A. thaliana to inoculation with Xanthomonas campestris pv. campestris (Lummerzheim et al. 1993, Molecular Plant-Microbe Interactions 6, 532-544), In addition to this phenotypic resemblance, accumulation of polyphenols and callose depositions observed by microscopic analysis, as well as increases in the activities of the stress-related proteins beta-1,3-glucanases, chitinases and peroxidases, revealed significant similarities in the plant response to the two treatments examined, lead toxicity and bacterial infection, The results allow the establishment of markers for both types of stress
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